The present invention relates to power supplies for use with gas-discharge display lamps. More particularly, the present invention relates to power supplies for use with neon lamps for us e in neon signs and other purposes. It should be understood that the term "neon lamp" is used herein to refer to all gas-discharge lamps and is not limited to lamps that contain only neon gas.
In many countries the maximum voltage relative to ground which may be produced by a neon lamp power supply is limited by local regulatory requirements for each of the countries. It is, therefore, common practice to use a power supply that produces a balanced output voltage with respect to ground, in which the balanced output voltage contains two high-voltage AC outputs that differ in phase by 180.degree.. FIG. 1 is a simplified circuit illustrating such a power supply 10, showing an AC current source 12 connected to a transformer 14 for powering a neon lamp or tube 16. The transformer 14 produces a balanced output voltage with respect to ground GND. Power supplies of this type may be used to power longer neon tube lengths than other conventional types of power supplies, without violating local regulatory requirements.
For a power supply that produces voltages above a certain defined minimum voltage, local regulatory requirements may also require that the power supply be provided with ground fault protection. The ground fault protection latches the power supply into an off state when current flowing to a ground return of the power supply exceeds a limit set by the local regulatory requirements.
An objective of having ground fault protection is to prevent an arc from the power supply to ground from igniting flammable materials and starting a fire. Ground fault protection may also be used as a safety measure for protecting humans from accidental injury by setting a ground fault current limit sufficiently low. Generally, such a ground fault current limit is at a level that is below typical current levels that a power supply would deliver to a neon tube.
One concern when using a neon tube power supply that operates at a high frequency in excess of 10 to 20 kHz is the stray capacitance produced in output leads connecting the power supply to the neon tube. Neon signs often include multiple neon tubes formed into a complex assembly of letters or artistic shapes and designs. A particular design of a neon sign may dictate that the output leads from the power supply have significantly different lengths. In addition, the neon sign itself may have significant capacitance between each of the neon tubes forming the neon sign and ground.
For example, FIG. 2 schematically shows a typical power supply 20 with a center-tapped transformer 24 driving a neon tube 26. The capacitors C.sub.s are shown in phantom to represent a sample distribution of stray capacitance to ground from the output leads 28, 30 of the power supply 20 and from the neon tube 26. Different lengths of output leads 28, 30 from the power supply 20 to the neon tube 26 will result in different values for the stray capacitance to ground of each output lead 28, 30, as indicated by a greater number of capacitors C.sub.s on the longer output lead 28 of FIG. 2.
In addition, a neon sign may have a particular layout that results in one end of the neon sign having a higher stray capacitance to ground than the other end of the sign. As shown in FIG. 3, if the neon sign 40 has an inner neon tube design 42 and an outer neon tube border 44, and the outer neon tube border 44 is located in close proximity to a grounded metal enclosure 46, the outer neon tube border 44 will have a higher stray capacitance to ground than the inner neon tube design 42, as indicated by the number of capacitors C.sub.s shown connected at various parts of the neon sign 40.
The effect of an unbalanced stray capacitance distribution, such as that shown in FIGS. 2 and 3, is to produce a capacitive current from the center tap of the transformer of the power supply to ground. This capacitive current can have a magnitude equal to or greater than the ground fault current limit or "trip point" at which the power supply is designed to automatically latch off in order to comply with local regulatory requirements. Therefore, the capacitive current produced as a result of stray capacitance can lead to "false trip" situations in which the power supply is latched off even though the capacitive current does not present the type of safety hazard as would an external arc.
To quantify the effect of the stray capacitance, values in excess of 50 picofarads have been observed in some commercial neon signs. If a neon sign has an imbalance or a difference in its capacitance to ground between two output leads of 35 picofarads, and the neon sign operates at 22 kHz and a voltage to ground of 3.5 kV RMS, then its ground fault current will be 16.9 mA. This current is above the trip level specified in UnderWriters Laboratories Specification UL2161, for example, which sets a limit for ground fault current of 15 mA.
Such a situation is not acceptable to users of high-frequency power supplies for neon lamps.